1. Field of the Invention
This invention relates to antipilferage systems and markers for use therein. In particular, it relates to such markers as produce a response to an alternating magnetic field.
2. Description of the Prior Art
Antipilferage systems relying on magnetic principles have long been known. Such systems are generally of two types, those such as are disclosed in U.S. Pat. Nos. 3,534,358 (Stern) and 3,559,201 (Hillard) which utilizes a marker comprising a nonmagnetic metallic foil such as aluminum, and those such as are disclosed in U.S. Pat. Nos. 3,292,080 (Trikilis) and 3,665,449 (Elder & Wright), which utilize a marker comprising a ferromagnetic material. In all such systems, the marker is essentially insensitive in at least one direction. A variety of schemes have been proposed to overcome this limitation: some provide a multidimensional shaped marker such as an L shape, while others provide multidirectional interrogating fields and sensors sensitive to fields along more than one axis. Systems using nonmagetic metal foils are still prone to false alarms resulting from the presence of other metallic objects such as briefcases, keys, etc. being carried through the interrogation zone. Systems based on magnetic markers have the disadvantage of being subject to false alarms due to the presence of extraneous magnetic materials.